An arrangement of this kind is known from German patent application DE 101 51 778 A1. In the arrangement described in that publication, first of all a magnetic selection field having a spatial distribution of the magnetic field strength is generated such that a first sub-zone having a relatively low magnetic field strength and a second sub-zone having a relatively high magnetic field strength are formed in the examination zone. The position in space of the sub-zones in the examination zone is then shifted, so that the magnetization of the particles in the examination zone changes locally. Signals are recorded which are dependent on the magnetization in the examination zone, which magnetization has been influenced by the shift in the position in space of the sub-zones, and information concerning the spatial distribution of the magnetic particles in the examination zone is extracted from these signals, so that an image of the examination zone can be formed. Such an arrangement has the advantage that it can be used to examine arbitrary examination objects—e.g. human bodies—in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object.
A similar arrangement and method is known from Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication takes advantage of the non-linear magnetization curve of small magnetic particles.
Generally, the drive means of such an MPI arrangement comprises drive field coil units and drive field generator units. For being flexible with the sequences, MPI needs a high reactive power provided by the drive field generator units. Commonly, a switched amplifier is used to generate the high reactive power. This amplifier stores the reactive energy in at the base band (i.e. at zero frequency or a frequency near zero) in a capacitor. To perform this, switching elements have to operate at frequencies in the order of the operation frequency, i.e. at 100 kHz and higher. The switching losses at that frequency are already relatively high. Moreover, the arrangement produces strong high harmonics which implies the use of large filters and even higher reactive power.
WO 2008/078244 A2 discloses an arrangement for influencing and/or detecting magnetic particles, a method for calibrating such an arrangement and a method for influencing and/or detecting magnetic particles in a region of action. The arrangement particularly comprises a compensation controller providing a compensation signal to the drive signal chain and/or to the detection signal chain by means of a coupling means to enhance the signal to noise ratio. One of the signal detection problems occurring in such an arrangement is the induced voltage in the receiving means due to the existence of the magnetic drive field. This induced voltage is typically much larger than the (useful and only interesting) signal voltage of the detection signal. The measurement principle of the arrangement relies on the fact that a magnetic drive field with a dedicated frequency influences the magnetic particles which send out a signal including this frequency but also higher harmonics. These harmonics are measured. Therefore, either the spectrum of the drive field itself must not contain higher harmonics of the dedicated frequency or the higher harmonics of the dedicated frequency have to be eliminated or compensated for—either in the so-called drive signal chain or in the so-called detection signal chain. Especially, it is preferred that the compensation signal is filtered prior to being coupled to the drive signal chain and/or to the detection signal chain. This provides the possibility to add the lowest possible portion of noise to the signals in the drive signal chain and/or the detection signal chain. In this context the term drive signal chain signifies the different stages—e.g. amplifying stage, filtering stage or the like—in order to generate the drive signal fed to the drive means. Likewise in the context of the present invention, the term detection signal chain signifies the different stages—e.g. amplifying stage, filtering stage or the like—in order to generate the detection signal received by the receiving means.